Blown film production lines are typically limited in output by bubble stability. Blending Linear Low Density Polyethylene (LLDPE) with Low Density Polyethylene (LDPE) increases bubble stability, in part due to the higher melt strength of the LDPE. The increase in melt strength in part provides for an increase in film output. However, too high a melt strength, especially as with broad molecular weight distribution (MWD), autoclave LDPEs with fractional melt indexes, can cause gels, limiting drawdown capabilities, which can result in poor quality films. Also, high melt strength LDPE resins typically have reduced optics. Thus, there is a need for new compositions containing ethylene-based polymers, such as tubular LDPEs, that have an optimized balance of melt strength, optics and mechanical properties, for blown film applications.
Linear Low Density Polyethylene (LLDPE) is typically more difficult to process on a blown film line with generally poorer bubble stability, or lower maximum output (mass/time such as pounds per hour) than Low Density Polyethylene (LDPE). Films made with Linear Low Density Polyethylene (LLDPE), however, generally have better film mechanical properties than those made with Low Density Polyethylene (LDPE). Film processing and film properties are to a large extent optimized for blown films by blending Linear Low Density Polyethylene (LLDPE) with Low Density Polyethylene (LDPE). Blending in lower amounts of LDPE into LLDPE typically leads to improved processing compared to pure LLDPE, improved optical properties, and acceptable mechanical properties. Blending in high amounts of LDPE into LLDPE improves processing further and allows thick film of a very large bubble diameter to be produced, while the mechanical and optical film properties are maintained or improved over film made from pure LDPE. LDPE-rich films are also especially suited to shrink films, such as collation shrink films, where the LDPE imparts good shrinkage behavior which cannot be achieved by the use of a LLDPE alone. In summary, the LDPE blend component contributes typically to the processability, optical properties, and shrink performance, while the LLDPE blend component contributes to the mechanical properties.
There is a need for new compositions that can increase the melt strength and the processing performance over conventional LDPE/LLDPE blends, and which can be made at low conversion costs in a tubular process. Furthermore, there is need for LDPE/LLDPE compositions with improved performance in processing (maximum line speed and or large bubble operation) and/or film properties (mechanical and shrink performance and/or optical appearance).
Low density polyethylenes and blends are disclosed in the following: U.S. Publication 2014/0094583; U.S. Pat. Nos. 5,741,861; 7,741,415; 4,511,609; 4,705,829; U.S. Publication No. 2008/0038533; JP61-241339 (Abstract); JP2005-232227 (Abstract); and International Publication Nos. WO2010/144784, WO2011/019563, WO 2010/042390, WO 2010/144784, WO 2011/019563, WO 2012/082393, WO 2006/049783, WO 2009/114661, US 2008/0125553, EP0792318A1 and EP 2239283B1. However, such polymers do not provide an optimized balance of high melt strength and improved film mechanical properties, for blown film applications. Thus, as discussed above, there remains a need for new ethylene-based polymer compositions that have an optimized balance of melt strength, optics, processability and output, and good shrinkage. These needs and others have been met by the following invention.